GFCI devices are designed to trip in response to the detection of a ground fault condition at an AC load. Generally, the ground fault condition results when a person or object comes into contact with the line side of the AC load and an earth ground at the same time, a situation which can result in serious injury. The GFCI device detects this condition by using a sensing transformer to detect an imbalance between the currents flowing in the line and neutral conductors of the AC supply, as will occur when some of the current on the line side is being diverted to ground. When such an imbalance is detected, an electrically-held relay having primary power contacts within the GFCI device is immediately denergized to place the primary power contacts in an open condition, thereby opening both sides of the AC line and removing all power from the load. Many types of GFCI devices are capable of being tripped not only by contact between the line side of the AC load and ground, but also by a connection between the neutral side of the AC load and ground. The latter type of connection, which may result from a defective load or from improper wiring, is potentially dangerous because it can prevent a conventional GFCI device from tripping at the intended threshold level of differing current between line and neutral when a line-to-ground fault occurs.
GFCI devices may be connected to fuse boxes or circuit breaker panels to provide central protection for the AC wiring throughout a commercial or residential structure. More commonly, however, GFCI devices are incorporated into electrical receptacles that are designed for installation at various locations within a building. A typical receptacle configuration is shown, for example, in U.S. Pat. No. 4,568,997, to Bienwald et al., the entire content of which is incorporated herein by reference. This type of receptacle includes test and reset pushbuttons and a lamp or light-emitting diode (LED) which indicates that the circuit is operating normally. When a ground fault occurs in the protected circuit, or when the test button is depressed, the GFCI device trips and an internal circuit breaker opens both sides of the AC line. The tripping of the circuit breaker causes the reset button to pop out and the LED to be extinguished, providing a visual indication that a ground fault has occurred. In order to reset the GFCI device, the reset button is depressed in order to close and latch the circuit breaker, and this also causes the LED to illuminate once again.
Ground fault protection from miswiring is also provided. Specifically, GFCI receptacles of the type described above may be erroneously connected with the incoming AC source conductors being tied directly to the load or feedthrough terminals of the receptacle rather than to the source terminals. Because of the nature of the internal wiring of the GFCI receptacle, this miswiring condition is not easily detected. AC power will still be present at the receptacle outlets, making it appear that the receptacle is operating normally. If the test push button is depressed, the latching mechanism within the GFCI receptacle will be released and the reset push button will pop out, again making it appear that the GFCI receptacle is operating normally and providing the desired ground fault protection. In reality, however, no such protection is being provided because the AC source has been wired directly to the receptacle outlets without passing through the internal circuit breaker of the GFCI device.
Furthermore, a user may not know the state the GFCI is in without having to operate the test and reset buttons. Therefore, a visual indication should be provided to indicate to a user the different states the GFCI is in.
As a GFCI device is repeatedly tested or is frequently interrupting the power-supply circuit, its primary contacts begin to wear and, over time, the primary contacts do not have sufficient area or contact pad left to effectively withstand the severe electrical consequences of interrupting an energized circuit, in particular, arcing. When the GFCI device primary contacts are at the end of their useful life, one of several possible failures is the welding together of the primary contacts. In this failure condition, the electrical power circuit to the load is not interrupted even though the GFCI circuit signaled that a fault has occurred and the circuit responded as designed (i.e., operated in a manner that would have normally opened the primary contacts), which normally would interrupt the electrical power circuit to the load. This is a potentially hazardous condition because the conventional GFCI device has signaled a fault, but the appliance or tool that may be causing the fault is still connected to a power supply. It is also possible that the conventional GCFI circuit would indicate that no fault has occurred, which would potentially harm a user through continued use of an appliance or tool that is in an unsafe condition.
Many GFCI devices employ a GFCI integrated circuit or chip in the sensing circuit that processes data received from the sensing transformers and provides an output or trip signal that can be used to activate a gated device such as an SCR and energize a solenoid and open the contacts. A microprocessor, in turn, monitors outputs from the GFCI chip and SCR, among other components. When ground fault (GF) current levels vary around the GFCI GF current threshold, intermittent GFCI chip outputs can occur. These intermittent outputs may have sufficient energy to turn the SCR on and, at the same time, result in insufficient energy in the solenoid coil to open the contacts. Thus, it is possible for the microprocessor to make a false EOL determination.
A need therefore exists for a GFCI device that can discriminate against intermittent GFCI chip output signals caused by GF current levels varying near the GFCI GF current threshold and thereby avoid a false or premature EOL determination.